Servo valves are well known in the art. These valves are typically used in hydraulic systems wherein a power supply, such as a pump, applies power to a load by means of a fluid circuit. A servo valve is the interface between the hydraulic system and an electrical, mechanical, fluid or other external type of controller. A single stage servo valve comprises an operating spool whose relative position in a ported valve body controls the rate pressure and direction of hydraulic fluid flow. Analog servo valves include internal feedback which can be electrical, force or mechanical in form.
Although analog servo valves are well known in the art, digital servo valve systems are a more recent development. An example of a digital servo valve can be found in the commonly owned U.S. Pat. No. 4,235,156. Disclosed therein is a single stage, spool type, four-way valve which is controlled by an electric DC stepping motor. The servo valve comprises a valve body with an interior cavity having a plurality of fluid channels. A valve operating member, including a spool, is slidable along a longitudinal axis of the interior cavity. Each of the fluid channels is ported to a respective element in the hydraulic circuit. A source of pressure (pump) communicates with a first channel. When the spool is moved along the axis, pressurizing fluid communicates through the fluid with the remainder of the hydraulic circuit, as is well known in the art. A digital controller is employed to provide control signals to a digital stepper motor which is connected to the operative member of the valve such that rotation of the stepper motor output shaft is translated to linear motion of the spool.
Unlike conventional servo valves, the servo valve disclosed in the '156 patent has no mechanical, electrical and/or force feedback. Mechanical servo valves include a provision for a feedback shaft which is operatively connected to the hydraulic load. The feedback shaft returns the spool to the closed or null position of the valve upon completion of the commanded operation. The servo valve disclosed in the '156 patent provides for true digital closed loop operation, and has no analog or mechanical feedback element. Rather, the necessary feedback is provided exclusively by a digital encoder equivalent or transducer that is directly connected to a hydraulic load. The encoder outputs a position feedback signal which is compared with a command signal by the digital controller. The controller provides pulses to the stepper motor dependent on the difference between the command and feedback signals.
As is well known, stepper motors are characterized by a plurality of magnetic detents which oppose output shaft rotation from its present position even if electrical power is removed from the stepper motor. Consequently, if the last command to the servo valve was for a valve open position, the valve will be in that open position when the valve is re-energized. This characteristic of digital servo valves presents a danger to both men and machinery, since the valve will be in an unknown state when electrical power is restored. On the other hand, an analog servo valve will inherently return to the fully closed position if the servo system loses electrical power because in most valves force feedback is used.
Another problem with known digital servo valves is the presence of mechanical backlash which occurs when the stepper motor changes its direction of rotation and displaces the spool in the opposite direction. To accomplish the necessary rotary to linear translation, a grooved helical cam and pin mechanism is configured at one end of the spool. In order to prevent backlash, a spring apparatus operatively connected to the spool exerts a biasing force against the spool. As a result, the pin is held against one of the sides of the cam groove, regardless of the direction of spool rotation. However, under certain conditions, this force can be overcome and produce the undesirable backlash.
Still another drawback of the prior art is apparent with larger servo valves. In general, flow forces developed within the valves increase with the size of the spool and valve body. These flow forces, or Bernoulli forces, oppose the operation of the valve from a null position. The torque required of the stepper motor to controllably displace the spool must correspondingly increase to avoid a situation where the flow forces developed within the valve are greater than the torque capability of the stepper motor. Larger servo valves of the prior art require larger, two-stage hydraulic amplification or more costly stepper motors. Alternatively, flow force can be reduced. However, to reduce flow forces, the prior art discloses a spool and valve body geometry with an extremely complicated "turbine bucket" shape, formed by relieving significant portions of the valve body. This valve body shape is extremely complex, with accompanying machining costs that are too great to be practical. It would be advantageous to have a digital servo valve configured so that flow forces within the valve are substantially reduced, enabling a smaller stepper motor to be used than would otherwise be possible and whose spool and chamber geometry is simple and inexpensive to manufacture.